SEOUL—South Korean officials threatened stern action Thursday against retail giant Costco in an increasingly heated row over the US firm’s refusal to shut its wholesale club stores on Sundays.

Nearly 40 Seoul council officials raided three Costco stores in the capital on Wednesday to crack down on minor parking and safety violations, in apparent reprisal for the retailer’s non-adherence to Sunday closing rules.Officials said they would conduct “more crackdowns” if Costco opened next Sunday.

In a bid to protect small mom-and-pop stores, the council imposed the Sunday closing rule on big-box discount chains in April.